Why did my just-rendered animation not save right?
It seems that your file got corrupted, somehow. I don't think there's anyway to fix this, unfortunately. In the future, you should render to image sequences, instead.

The nice advantage of doing it this way is that even if, for whatever reason, Blender fails to render, not all will be lost; you can just continue rendering.

To do this, just set the output format to either:


JPEG (small, lossy file format with low color depth),
PNG (small, lossless file format with low color-depth) or 
EXR (huge, lossless file format with high color-depth) 


Make sure to select an appropriate output folder, for if you save to a temporary folder, your OS may decide to delete them at any point.



Then, once you're done rendering, you can switch to the Video Sequence Editor, by selecting Video Editing from the dropdown at the top:



Afterwards, add your image sequence by clicking Add> Image, found in the toolbar bellow, and selecting them from the folder.



After this, you can switch back and select a video format and press Render. This time, it will not render the scene, but only assemble the image sequence into a video format.